The present invention relates to latching mechanisms, and more particularly to latching mechanisms for securing accessories in a radiation therapy system.
Radiation therapy systems for the treatment of cancer are well known in the art. FIG. 1 illustrates one such system manufactured by Siemens Medical Systems, Inc.(trademark). The system 10 includes a platform 20 onto which a cancer patient lies. The rotating gantry 30 administers the radiation to the patient. The rotating gantry 30 includes an accessory holder 100 onto which accessory trays (not shown) may be inserted. The trays help an operator aim the radiation beam onto the patient. For example, a tray may contain a block or a filter to vary the concentration of the radiation beam. The tray may also have a hole patterned to match the shape of a patient""s tumor to help focus the radiation beam. The trays are made of metal, typically one to two inches thick.
FIGS. 2A and 2B illustrate a top view and a cross-sectional view, respectively, of an accessory holder for a conventional radiation therapy system. The accessory holder 100 is herein described with simultaneous reference to FIGS. 2A and 2B. The accessory holder 100 comprises a frame 102 which contains three slots 104, 106, 108 into which accessory trays, such as tray 110, may be inserted. The trays are latched to the frame 102 by latching mechanisms 112, 114, 116. Each of the latching mechanisms 112 and 114 comprises a lock 118. The lock 118 has a head 120 at one end and a protrusion 122 at the side of the lock 118 nearer the tray 110. A roll pin 124 couples the lock 118 to a release assembly 126. Coupled at the opposite end of the release assembly 126 is a release button 128 which when pressed, compresses a button spring 130.
The latching mechanisms 112 and 114 are coupled to the frame 102 by a retainer assembly 132 which includes a retainer shaft 134. When the tray 110 is inserted into the slot 104, the protrusion 122 of the lock 118 engages a notch 136 on the tray 110, preventing the tray 110 from sliding out of the slot 104. An opening 154 in the frame 102 allows the lock 118 to freely engage the notch 136. When the release button 128 is pressed, the release assembly 126 pivots the lock 118 counterclockwise about the retainer shaft 134, releasing the tray 110. This is facilitated by the wedging action between an elongated hole 152 of the release assembly 126 and the roll pin 124. The release button 128 is spring loaded. This button spring 130 keeps the lock 118 engaged when a tray is inserted. A crescent washer 138 is used to constrain the latching mechanisms 112 and 114 in the direction of the retainer shaft 134 axis. The compliance of the crescent washer 138 limits the friction in the latching mechanisms 112, 114 while maintaining precision. The spacer 140 maintains the proper height alignment between the latching mechanisms 112 and 114 and their respective slots 104 and 106. The latching mechanism 116 for slot 108 uses a retainer ring 142 to loosely constrain the parts. The release assembly 144, the roll pin 146, the lock 148, and the spacer 150 for latching mechanism 116 perform the same functions as those for latching mechanisms 112 and 114.
However, the dislodgment of the roll pin 124 and/or the loss of release button spring 130 force due to breakage of the spring 130 or loosening of the release button 128 have been known to occur. If one of these failures occur, then the tray 110 is in danger of sliding out of the accessory holder 100 and falling onto a patient on the platform 20, possibly causing serious injuries.
Accordingly, what is needed is a system and method for a secondary lock for securing accessories in a radiation therapy system. The secondary lock should decrease the possibility of a failure of the accessory holder""s latching mechanism. The present invention addresses such a need.
The present invention provides a method and system for a secondary lock for securing accessories in a radiation therapy system. The latching mechanism provided in accordance with the present invention includes a primary lock, a secondary lock, and a retainer shaft. The primary lock includes a head at a first end of the primary lock, and a protrusion at a first side of the primary lock. The secondary lock includes a main body, an engagement tang at a first end and a first side of the main body, and a locking tang at the first end and a second side of the main body. The retainer shaft is coupled to the primary lock and the main body of the secondary lock, where the secondary lock is fixedly positioned when the protrusion engages the accessory and the head resides between the engagement tang and the locking tang. The secondary lock creates a fail-safe locking system, such that the secondary lock causes the primary lock to continue to engage an accessory tray even if failure occurs elsewhere in the latching mechanism. This lowers the probability of failure of the latching mechanism significantly, resulting in a safer radiation therapy system. The secondary lock of the latching mechanism also remains in a disengaged position when no tray in inserted, allowing trays to be inserted without the need to press the release button first to set the secondary lock into the disengaged position.